The invention is related to improvements in sifters for sifting granular material, and more particularly to a sifter such as used in the manufacture of cement wherein material is to be separated into a coarse grain, a medium and a fine grain and the more coarse grains supplied with raw material through an interparticle crushing ball mill.
In the development of grinding systems, particularly such as used in the crushing of brittle grinding stock such as cement clinker, a unique method and apparatus has been accepted for its improved energy-saving and product production which operates under the principles sometimes known as interparticle crushing. The principles of interparticle crushing are disclosed and exemplified in European Patent 0 084 383 and in U.S. Pat. Nos. 4,357,287, Schoenert, and 4,703,897, Beisner et al.
In the preprocessing of pulverulent material, particularly when using the principles of interparticle crushing, sifters are employed to separate the pulverulent material into different fractions ranging from fine to coarse grain fractions in order to obtain optimum handling of the material for crushing and to obtain optimum uniformity and to achieve the reduction of energy required to convert the material from the brittle grinding stock nature to completed fine material such as where cement clinker is converted into finished cement.
A known sifter (U.S. Pat. No. 2,973,094) comprises a housing having two chambers in each of which a rotating rod basket provided with turbo elements is rotatably seated. A coarse grain fraction is separated in the first chamber from a material to be sifted which is introduced thereto together with the sifting airstream and a medium grain fraction is separated in the second chamber, after which the fine material contained in the material for sifting leaves the sifter together with the sifting airstream. The rotating rod baskets come into intense contact with the material for sifting when sifting the various grain fractions. It is self-evident that the rod-shaped turbo elements of the rod basket rotating in the first sifter chamber are particularly subject to high wear as a consequence of the impact contact by the coarse grains of material. This is even more the situation when the material to be sifted in the sifter is the discharged material of a high-pressure interparticle crushing roller press. The energy-saving interparticle crushing of brittle grinding stock is disclosed, for example, for cement clinker in European Patent 0 084 383 and U.S. Pat. No. 4,357,287. In interparticle crushing, namely, the individual particles of the grinding stock drawn in the nip by friction are mutually crushed in a product bed, i.e. in a material fill compressed between the two roller surfaces of the roller press with the application of an extremely high pressure, and are pressed to form agglomerates of material, what are referred to as scabs. These are composed of material grains having a comparatively sharp-edge grain surface which highly intensify the wear of a following sifter, particularly when the latter is a dynamic sifter having at least one rotating rod basket.
An object of the invention is to create a simply constructed sifter that is suitable for being utilized in a circulating grinding system with a high-pressure interparticle crushing roller press, whereby the sifter is a dynamic sifter having at least one rotating rod basket that, however, is nonetheless not subject to high wear.
A further object of the invention is to provide an improved sifter structure which is capable of improved sifting operation and in particular removes coarse grain fraction material before such material can engage the rotary sifter.
A further object of the invention is to provide an improved arrangement constituting a sifter arrangement in combination with an interparticle crushing roller press which is capable of sifting and pressing the material in an improved sequence such that an improved more uniform product results and improved energy-saving results with a reduction in wear of the equipment.